The restoration of teeth commonly involves a light curable dental composition containing free-radically polymerizable resins. Light curing of a dental composition involves a photoinitiator system generating free radicals upon exposure to visible light. Free radicals may be typically produced by either of two pathways:    (1) the photoinitiator compound undergoes excitation by energy absorption with subsequent decomposition of the compound into one or more radicals (Norrish type I), or    (2) the photoinitiator compound undergoes excitation and the excited photoinitiator compound interacts with a second compound by either energy transfer or a redox reaction to form free radicals from any of the compounds (Norrish type II).
In order for a photoinitiator to be useful for use in a dental composition, the quantum yields indicating the conversion of light radiation to radical formation needs to be high since absorption or shielding of light by further components of the dental composition limit the amount of energy available for absorption by the photoinitiators. Accordingly, only about 70 percent conversion of the polymerizable groups may be expected in a polymerization of a typical dental composition, whereby the mechanical strength of the polymerized dental composition is less than optimal and unreacted monomers may leach out of the the polymerized dental composition. The leaching monomers may have detrimental effects. In order to alleviate this problem, multifunctional monomers are frequently used which are more likely to be included in the polymer network.
In addition, photoinitiators are required to have a high acid resistance, solubility, thermal stability, and storage stability when incorporated into a dental composition.
Finally, given that dental compositions usually contain (meth)acrylate or (meth)acrylamide monomers, free radical photocuring may be inhibited by the presence of oxygen. Oxygen inhibition is due to the rapid reaction of propagating radicals with oxygen molecules to yield peroxyl radicals which are not as reactive towards carbon-carbon unsaturated double bonds and therefore do not initiate or participate in any photopolymerization reaction. Oxygen inhibition may lead to premature chain termination and, therefore, incomplete photocuring. Nevertheless, a certain degree of oxygen inhibition on the top surface of the adhesive layer is required for the bonding to the adjacent restorative.
Accordingly, the polymerization initiator system has a critical influence on the quality of the dental material. Conventionally, camphor quinone optionally in combination with a tertiary amine, or 2, 4, 6-trimethylbenzoylphenyl phosphinate (Irgacure® TPO) are frequently used as photoinitiator system. However, the presence of amines in acrylate-containing compositions can cause yellowing in the resulting photocured composition, create undesirable odors, and soften the cured composition because of chain transfer reactions and therefore, often require the use of stabilizers. Moreover, the use of aromatic amines gives rise to toxicological concerns.
Furthermore, it is desirable that the light activating the photoinitiator system has a long wavelength in order to avoid damage of soft tissue during polymerization of the dental composition in the patient's mouth. Accordingly, the photoinitiator system is required to contain a chromophoric group efficiently absorbing light of the desired wavelength in a range of from 400 to 800 nm. However, an increase of the absorption coefficient of the photoinitiator system increases the coloration of the photoinitiator system and thereby the coloration of the dental composition before light curing. Accordingly, it is necessary that the chromophoric groups are efficiently destroyed during polymerization so that the coloration of the initiator system disappears in the polymerized dental composition, the so-called “photo-bleaching”. A destruction of the chromophoric groups during polymerization may also be useful in increasing the depth of cure of the dental composition since activating light is not shielded from unpolymerized layers of the dental composition by the photoinitiator system present in polymerized layers covering the unpolymerized layers.
U.S. Pat. No. 3,534,122 discloses a free radical polymerizable composition containing liquid monomeric acrylate and methacrylate esters of monohydric and polyhydric alcohols. A tertiary organo-phosphine promoter is disclosed for accelerating polymerization chemically initiated by mono-tertiary butyl peroxy permaleate.
WO 2009/147033 A1 discloses a photoinitiator mixture comprising at least one specific alpha-amine ketone compound and at least one specific oxime ester compound. WO 2012/045736 A1 discloses benzocarbazole derivatives as photoinitiator. WO 2014/060450 A1 discloses combinations of phenylglyoxylic acid compounds with alpha-hydroxyketones as photoinitiators. Phosphorus compounds, for example triphenylphosphine, are mentioned as additive for increasing the stability on storage in the dark.
U.S. Pat. No. 5,545,676 discloses a ternary photoinitiator system for addition polymerization which comprises an aryliodonium salt, a sensitizing compound and a specific electron donor. Triphenylphosphine is disclosed in combination with CQ. Specifically, U.S. Pat. No. 5,545,676 discloses a composition obtained by adding equimolar amounts of triphenylphosphine were to a monomer stock solution containing 50 parts trimethylolpropane trimethacrylate, 50 parts 1,4-butanediol dimethacrylate, 0.25 part CPQ and optionally 0.5 part of the iodonium salt diphenyliodonium hexafluorophosphate. Triphenylphosphine is not preferred for lack of an abstractable hydrogen atom on a carbon or silicon atom alpha to the donor atom, which is reflected by the experimental results reported.
U.S. Pat. No. 6,187,836 discloses compositions featuring cationically active and free radically active functional groups. U.S. Pat. No. 6,187,836 discloses triphenylphosphine as cationic polymerization modifier. Specifically, U.S. Pat. No. 6,187,836 discloses a composition obtained by combining 10.0 g of a stock solution with a sufficient amount of a cationic polymerization modifier to achieve a modifier concentration of 1.13×10−4 moles per 10.0 g of stock solution wherein the stock solution is obtained by combining 5.0 g camphorquinone (CPQ) and 15.0 g diaryliodonium hexafluoroantimonate (CD1012 from Sartomer) with 720.0 g Cyracure® UVR 6105 cycloaliphatic diepoxide resin (available from Union Carbide), 180.0 g of a polytetrahydrofuran diol having an average molecular weight of 250 (p-THF-250, available from Aldrich Chemical Co.), and 100 g of acrylate oligomer (Ebecryl 1830, available from UCB Radcure, Inc.)), and stirring until homogeneous under safe light conditions.